1. Field of the Invention
The invention relates to a for recording apparatus for recording an information signal in a track on a record carrier, the information signal comprising packets that may occur irregularly as a function of time in the serial datastream of the information signal, the apparatus comprising
input means for receiving the information signal,
time stamp generator means for generating time stamps having a relationship to the moment of occurrence of a packet comprised in the information signal received, the time stamp generator means being adapted to generate subsequent cycles of count values in response to a clock signal, the time stamp generator means being adapted to derive a time stamp value in response to the detection of occurrence of a packet in said information signal received, a time stamp value for said packet having a relationship to the count value at said moment of occurrence of said packet,
combining means for combining a packet and its corresponding time stamp value so as to obtain a composite packet, and
writing means for writing the composite packet in said track on the record carrier, to a reproducing apparatus and to a recording method.
2. Description of the Related Art
A recording apparatus as defined in the opening paragraph is known from WO 96/30905 (PHN 15,260), corresponding to U.S. patent application Ser. No. 08/622,508, U.S. Pat. No. 5,953,483, filed Mar. 26, 1996. The apparatus is adapted to record an MPEG encoded information signal, such as a video signal, on a record carrier, such as a magnetic record carrier.
In prior art systems for recording MPEG Transport Streams, it is necessary to lock a local timestamp counter to the Program Clock Reference of the incoming Transport Stream during recording. The timestamp counter is used to record the arrival time of an incoming Transport Stream packet in order to be able to reconstruct the original packet timing during playback. The timestamp counter must be locked to the Program Clock of the incoming stream to make the recording independent of any frequency offset in the incoming Program Clock Reference signal. In addition, The Program Clock Reference of the incoming stream can be used as a timing reference for processes that need to lock to it (e.g., a drum in D-VHS). During playback, a free running clock is needed with a frequency of 27 MHz and an accuracy of for instance, +/xe2x88x9220 ppm. From this clock, the packet timing is reconstructed again and processes may be locked to this clock.
The invention aims at providing an improved recording apparatus. The recording apparatus in accordance with the invention is characterized in that the time stamp generator means comprises:
variable increment value determining means for generating a variable increment value in response to a control signal,
adder means for adding the variable increment value to a count value in response to the clock signal so as to obtain a subsequent count value in a cycle of count values, and
comparator means for comparing one or more program clock reference values comprised in packets in the information signal with one or more time stamp values so as to derive the control signal therefrom.
The invention is based on the following recognition. Normally, in the prior art recording apparatuses, the timestamp counter runs on a local clock whose frequency is locked to the Program Clock Reference of the incoming stream. Any processes that need to be locked to the Program Clock Reference can derive their timing from the locked clock. The local clock is derived from a Voltage Controlled Crystal Oscillator (VCXO) which is adjusted by a feedback control loop. During playback, the VCXO is left free running (it gets a fixed control voltage) and its frequency should stay as close as possible to 27 MHz.
To do this, two conflicting requirements are present for the VCXO. On the one hand, during recording, it needs to be pullable in order to lock to the incoming Program Clock Reference, while on the other hand, it needs to be stable and stay as close as possible to 27 MHz during playback.
In accordance with the invention, a solution is presented that enables the locking with a crystal that does not need to be pulled and only has requirements for stability. Instead of locking the frequency of the clock during recording, only the increment speed of one or more counters is adjusted (locked) to the speed of the incoming Program Clock Reference. The state of the locked counter(s) is used as a reference inside the recording system.